Fixed test failures with embedded server

mysql-test/mysql-test-run.pl:
  Don't set --log-error when running embedded server as we don't want the mysqltest output into mysqld.1.err
sql/mysqld.cc:
  Allow one to disable --log-error

mysql-test/mysql-test-run.pl

@@ -1397,6 +1397,9 @@ sub command_line_setup {
     # Turn off use of bin log
     push(@opt_extra_mysqld_opt, "--skip-log-bin");
 
+    # Write errors to stderr, not to mysqld.1.err
+    push(@opt_extra_mysqld_opt, "--disable-log-error");
+
     if ( using_extern() )
     {
       mtr_error("Can't use --extern with --embedded-server");

sql/mysqld.cc

@@ -8691,7 +8691,7 @@ mysqld_get_one_option(int optid,
     break;
   }
   case (int) OPT_ERROR_LOG_FILE:
-    opt_error_log= 1;
+    opt_error_log= (argument != disabled_my_option);
     break;
 #ifdef HAVE_REPLICATION
   case (int) OPT_INIT_RPL_ROLE: